The invention relates to a conveyor unit for a book carrier in electrophotographic copying machines, and more particularly, to such conveyor unit used in an electrophotographic copying machine of the type in which an original to be copied is fed through an exposure station for the purpose of copying and in which the conveyor unit operates to convey a book carrier carrying an original having a substantial thickness such as a book thereon through an exposure station of the copying machine.
As is well recognized, a conventional electrophotographic copying machine of the type in which an original is fed through an exposure station for copying purpose is constructed as illustrated in FIG. 1, for example. The machine is provided with an original feed path which is normally conditioned to enable the copying of a single sheet-shaped original, as shown. In the condition shown, a sheet-shaped original 1 is placed on an inclined original receptacle 2, and is inserted into the inlet of an original conveyor unit which includes pairs of conveyor rollers 4A, 4B and 5A, 5B and a pair of guide plates 6, 7. After passing through the inlet, the original 1 is fed into the nip between the pair of vertically spaced conveyor rollers 4A, 4B to be fed toward and through an exposure station 8 while passing between the guide plates 6, 7 and between the guide plate 6 and the exposure station 8. After passing through the exposure station 8, the original 1 is fed into the nip between the other pair of vertically spaced conveyor rollers 5A, 5B to be delivered onto an original tray 9.
As the original 1 is fed by the original conveyor unit, a pair of microswitches 3A, 3B, located adjacent to the rollers 4A, 4B, detect the position of the original 1, and the timing of operation of the various parts of the electrophotographic copying machine is controlled based upon this detection. As the original 1 passes through the exposure station 8, an illumination lamp 10 illuminates the surface of the original, whereupon an exposure optical system 11 projects an image of the original 1 onto a photosensitive drum 12. The drum 12 rotates in a direction indicated by an arrow B. Any electric charge is initially removed from the drum surface by means of a neutralizer lamp 13, and then the drum surface is uniformly charged by means of a corona charger 14. Then the drum surface is irradiated with the light image of the original to have an electrostatic latent image of the original 1 formed thereon. The latent image is developed by a developing unit 15 of dry type to form a toner image, which is moved to a transfer station 16 as the drum 12 rotates.
On the other hand, a record sheet 18 is fed one by one from its stack contained in a cassette 17 by means of a rocking and rotating feed roller 19, and is fed to the transfer station 16 by a pair of vertically spaced feed rollers 20 in timed relationship with the rotation of the drum so that the record sheet is superimposed on the toner image on the drum surface. Thereafter, the record sheet is conveyed between the drum 12 and a transfer roller 21 to which a bias voltage is applied. Such process transfers the toner image onto the record sheet. Since the record sheet is conveyed in tight contact against the drum surface during the process, the sheet must be separated from the drum surface by utilizing a separation claw 22 which cooperates with an airstream, as will be described later. After the transfer step, the record sheet is conveyed along a guide plate 23 to be fed, by a pair of vertically spaced feed rollers 24, into a fixing unit 25 which includes a heater where the toner image is fused and fixed to the record sheet. Subsequently, the sheet is delivered onto a copy tray 27 by means of a pair of vertically spaced delivery rollers 26.
Any residue of toner which remains on the drum surface after the transfer step is removed by a rotating cleaning brush 28, from which toner is withdrawn by an airstream created by a fan 29 so as to be collected in a filter 30. Both the cleaning brush 28 and the fan 29 are covered by a casing 31 in order to produce an effective withdrawal effect upon the toner residue and to prevent a dispersion of the toner into the apparatus. An airstream displaced by the fan 29 is introduced into a duct 32 having its outlet port 32a located adjacent to the transfer station 16, so that the airstream is effective to separate the record sheet from the drum 12 by cooperation with the separation claw 22.
The disclosed copying machine is designed so that an electrostatic latent image once formed on the drum surface may repeatedly be used to provide a plurality of copies in succession through a repeated process of developing with a toner and image transfer. In this instance, the cleaning brush 28, which is mounted on a holding member 34 rotatably mounted on a support shaft 33, is moved away from the drum 12 and the neutralizing lamp 13 and the charger 14 are maintained inoperative.
Such copying machine is normally used to provide a copy or copies from a single sheet-shaped original, but can be operated to provide a copy from an original having an increased thickness such as a book in a manner to be mentioned below. Specifically, a frame 35 which is adapted to define an extension of a conveying path and carrying the upper conveyor rollers 4B, 5B, which are constructed as follower rollers, and also carrying the guide plate 6 is pivotally mounted on a pin 36 so as to be turned through 180.degree. in a direction indicated by an arrow C. When so turned, the rollers 4B, 5B and the guide plate 6 define as extension of the conveying path which is contiguous with the exposure station 8 and which is located above the original tray 9 (see FIG. 2).
A book carrier for placing a thick original can now be used. A book carrier having a thick original placed thereon is initially placed on the original receptacle 2, and is conveyed toward the exposure station 8 by means of conveyor rollers 4A, 5A, which act as drive rollers during a copying operation and which cooperate with their associated drive members. As the book carrier passes through the exposure station, the original is irradiated through a transparent plate on which it is placed, whereby a copying operation takes place.
In the prior art practice, the book carrier has been conveyed by disposing the bottom surface of the book carrier in contact with the drive rollers 4A, 5A. Since the drive depends on the force of friction acting between the bottom surface of the book carrier and drive rollers 4A, 5A, the arrangement essentially fails to provide a stabilized speed, thus disadvantageously causing "step out" in the copied image.
To overcome this difficulty, in practice, a user had to place his hand on the book carrier to press it down while it is being conveyed until it bears against the first drive rollers 4A. The hand must be continued to be placed against the book carrier to maintain the abutment of the book carrier against the drive rollers 4A and the following drive rollers 5A.
Such use of the book carrier is troublesome or tedious and requires a certain degree of skill, since if the book carrier is strongly pressed down, the pressure applied may exceed the force of friction to cause the drive rollers to slip, thus ceasing a drive transmitted to the book carrier. Alternatively, the pressure applied may cause a flexure in the transparent plate of a reduced thickness on which the book carrier is placed, causing it to move into contact with guide members of the copying machine rather than the drive rollers 5A, 5B, thus causing a damage which disadvantageously results in a degradation in the copied image, for example, a whitening of a black image area or letters becoming thin.
There is also known a conveyor unit in which follower rollers are disposed so that their opposite lateral edges are in opposing relationship with the lateral edges of drive rollers to hold a book carrier formed by a transparent plate therebetween as the book carrier is being conveyed. In this construction, the conveying operation of the book carrier is initiated when the book carrier is inserted into the nip between the initial drive roller and its associated follower roller. However, the book carrier must be pushed into the nip against the pressure exerted by the follower roller before the book carrier can be conveyed since it must be held between the cooperating rollers under the pressure exerted by the follower roller. Hence, the initial insertion of the book carrier into the nip between the rollers is difficult, requiring a degree of skill for a proper operation.
In this record arrangement, again the conveying operation of the book carrier takes place as a result of the force of reaction acting between it and the drive and follower rollers, inevitably causing an unstable conveying speed and the likelihood to cause the aforementioned "step out" condition. In addition, when the book carrier is being conveyed by being held between the drive and follower rollers under pressure, the book carrier or the transparent plate may still move into abutment against guide plates to be damaged or flexed to cause an upward shift at the exposure station, preventing proper focussing within the optical system from being achieved.
To stabilize or assure a positive insertion of the book carrier into the nip between the rollers, the positional relationship between, the outer diameters, the material and hardness of the drive and follower rollers must be closely controlled, presenting a serious quality maintenance problem and an increased cost.